The present invention relates to an air-fuel ratio control system for an internal combustion engine mounted on a vehicle, which controls the air-fuel ratio of an air-fuel mixture to an approximate stoichiometric air-fuel ratio value at which a three-way catalyst acts most effectively.
In a conventional air-fuel ratio control system, the air-fuel ratio of the air-fuel mixture burned in cylinders of the engine is detected as oxygen concentration in exhaust gases by means of an O.sub.2 sensor provided in an exhaust system of the engine, and a decision is made from the output signal from the O.sub.2 sensor whether the air-fuel ratio is richer or leaner than the value corresponding to the stoichiometric air-fuel ratio for producing a control signal. The control signal is applied to a proportion and integration circuit (PI circuit), the output of which is changed to pulses. The pulses operate an electromagnetic valve for controlling the air-fuel ratio of the mixture. Thus, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio at which the three-way catalyst acts most effectively. In such an air-fuel ratio control system, when the vehicle is accelerated or decelerated the air-fuel ratio is subject to deviate from the stoichiometric air-fuel ratio. Japanese patent application laid open No. 51-124738 discloses an air-fuel ratio control system which is provided with detecting means for detecting transient conditions such as rapid acceleration and deceleration and for correcting a feedback control signal so as to make the deviated air-fuel ratio converge to the stoichiometric air-fuel ratio. The detecting means comprises a throttle position sensor or an intake manifold vacuum sensor. Such a system of the prior art has a disadvantage that feedback operation is delayed when the air-fuel ratio greatly deviates at transient.